Dynamic bath control process

ABSTRACT

A process for maintaining a continuous and stable aqueous nickel sulfamate electroforming solution adapted to form a relatively thin, ductile, seamless nickel belt by electrolytically depositing nickel from said solution onto a support mandrel and thereafter recovering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprising: establishing an electroforming zone comprising a nickel anode and a cathode comprising said support mandrel, said anode and cathode being separated by said nickel sulfamate solution maintained at a temperature of about 140* to 160* F and having a current density therein ranging from about 200 to about 500 amps/ft2; imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution; maintaining said solution within said zone at a stable equilibrium composition: Total Nickel 12.0 to 15.0 oz/gal Halide as NiX2.6H2O 0.11 to 0.23 moles/gal H3BO3 4.5 to 6.0 oz/gal ELECTROLYTICALLY REMOVING METALLIC AND ORGANIC IMPURITIES FROM SAID SOLUTION UPON EGRESS THEREOF FROM SAID ELECTROFORMING ZONE: CONTINUOUSLY CHARGING TO SAID SOLUTION ABOUT 1.0 TO 2.0 X 10 4 MOLES OF A STRESS REDUCING AGENT PER MOLE OF NICKEL ELECTROLYTICALLY DEPOSITED FROM SAID SOLUTION; PASSING SAID SOLUTION THROUGH A FILTERING ZONE TO REMOVE ANY SOLID IMPURITIES THEREFROM; COOLING SAID SOLUTION SUFFICIENTLY TO MAINTAIN THE TEMPERATURE WITHIN THE ELECTROFORMING ZONE UPON RECYCLE THERETO AT ABOUT 140* TO 160* F. at the current density in said electroforming zone; and recycling said solution to said electroforming zone.

ilite ttes atent 1 Bailey et a1.

DYNAMIC BATH CONTROL PROCESS Inventors: Raymond E. Bailey; Douglas A. Kreckel, both of Webster, NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: May 21, 1973 Appl. No.: 362,592

Related US. Application Data Continuation-in-part of Ser. No. 251,045, May 8, 1972, abandoned.

US. Cl 204/9, 204/238, 204/239 lnt. Cl C231) 7/02, BOlk 3/00 Field of Search 204/9, 237-241,

References Cited UNITED STATES PATENTS 6/1942 Norris 204/9 6/1965 Gelfand et al. 204/238 11/1969 Lundquist 204/9 3/1972 Morawetz et al. 204/238 Primary Examiner-T. M. Tufariello ABSTRACT said solution onto a support mandrel and thereafter recovering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprising: establishing an electroforming zone comprising a nickel anode and a cathode comprising said support mandrel, said anode and cathode being separated by said nickel sulfamate solution maintained at a temperature of about 140 to 160 F and having a current density therein ranging from about 200 to about 500 ampslft imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution; maintaining said solution within said zone at a stable equilibrium composition:

Total Nickel 12.0 to 15.0 oz/gal Halide as NiX,.6l-1 O 0.1 l to 0.23 moles/gal H 80 4.5 to 6.0 oz/gal electrolytically removing metallic and organic impurities from said solution upon egress thereof from said electroforming zone: continuously charging to said solution about 1.0 to 2.0 X 10 moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution; passing said solution through a filtering zone to remove any solid impurities therefrom; cooling said solution sufficiently to maintain the tem perature within the electroforming zone upon recycle thereto at about 140 to 160 F. at the current density in said electroforming zone; and recycling said solution to said electroforming zone.

19 Claims, 1 Drawing Figure 22 34 30 r /6\ /4 l /2 2o- 22 SOLUTION ELECTRO' ELECTRO- COOLING RECOVERY A UF PURIFICATION SUMP l l I I l l BELT FORMING SOLUTION TREATING 1 LOOP LOOP l 1 l CLEANING a l HEAT PARTlNG PREHEAT EXCHANGE F'LTER DYNAMIC BATH CONTROL PROCESS This application is a continuation-in-part of copending application Ser. No. 251,045, filed on May 8, 1972 and now abandoned.

This invention relates to a continuous nickel electroforming process. More particularly this invention relates to a process for maintaining steady state conditions in a dynamic nickel sulfamate electroforming bath which, in turn, enables the obtainment of a continuous, high output, nickel electroforming process. This process is useful for the fabrication'of endless nickel belts which serve as supports for electrically photosensitive image retention surfaces employed in electrostatographic reproduction apparatus In the field of electrostatographic reproduction, an image is reproduced by initially establishing an electrostatic latent image on an image retention surface. The image retention surface typically comprises a layer of electrically photosensitive material such as vitreous selenium formed on an electrically conductive substrate. The electrostatic latent image can be developed by contacting the surface with a developer material which comprises, for example, a pigmented, electroscopic, thermoplastic resin. This developer material adheres to the photosensitive surface in image configuration and can be subsequently transferred to a record medium such as a web or sheet of paper to which it is then fixed.

In commercial practice, the photosensitive material is generally deposited on a conductive substrate generally comprising a cylindrically shaped body or drum.

which is rotated about an axis thereof through a series of stations wherein charging, imaging, developing, image transfer and fixing operations occur. Although apparatus incorporating a drum shaped support body has met with wide commercial acceptance, the use of a drum introduces certain limitations in the design, construction and operation of the apparatus. One means of overcoming these limitations is through the use of a flexible substrate support body formed as a band or an endless belt. However, an endless belt support body for use with an electrostatographic reproduction apparatus should, in addition to being electrically conductive and flexible, be seamless in order to avoid the necessity for indexing the operation of the machine to inhibit image formation at a seam of the belt.

An endless, seamless belt suitable for use in an electrostatographic apparatus must exhibit relatively high tensile strength to withstand the stresses imposed thereon during use and to avoid yielding under such conditions causing slack in the system which gives rise to vibrations and improper tracking. It has been found that for a belt of about 5 mils thickness, for example, the tensile strength must be at least about 90,000 psi for proper operation. Of course, as the thickness of the belt decreases, the tensile strength thereof must increase commensurately. Also, the belt must be sufficiently ductile to easily flex over rolls used to effect rotation of the belt without destroying the substrate due to the stresses encountered in flexing, rotation and translational motion of the belt. Moreover, there must be sufficient ductility to enable flexing without destroying the integrity of the photosensitive surface. Ductility in the belt (as measured by elongation) of from about 3 to 12 percent has been found suitable.

In addition to physical property requirements, surface characteristic requirements must also be met. Surface flaws such as pitting, nodular spotting, and other localized surface defects, such as arise from replication of the mandrel surface or suspended particulate materials, must be kept to a minimum because the effect thereof is manifested in poor ultimate copy quality. Thus, any observable pit or any nodular spot having a diameter of about 10 mils or greater in the belt is sufficient to result in rejection of that belt.

It has also been found that overall surface roughness as distinguished from localized surface defects must be kept within limits in order to effect acceptable adhesion of the photosensitive material to the belt and yet not be so high as to interfere with the coating process used to deposit the photosensitive material on the belt. Surface roughness as used herein is determined by the standards set forth by the American Society of Mechanical Engineers ASA B46.ll962. It has been found that suitable belts exhibit a surface roughness ranging from about 10 to 80 microinches, RMS and preferably about 30 to 50 microinches, RMS. As the surface roughness decreases below about 50 microinches, the adhesion of the photosensitive material to the belt may become insufficient to withstand the mechanical and thermal shocks normally encountered in handling. As the surface roughness increases above about 60 microinches, RMS, coating difficulties are encountered because the effective increase in surface area of the belt at high surface roughness values results in coating of the photosensitive material onto the belt under cooler conditions since the belt acts as a heat sink under such conditions resulting in a frosted appearance in the photoreceptor which interferes with inspection of the belts for surface flaws. If the temperature of the coating operation is raised to overcome this of this type is disclosed in copending US. patent appliproblem, pitting in the resultant coating is obtained. Thus, control of surface roughness is important to the successful obtainment of suitable photosensitive belts.

These are few known methods suitable for production of such large, thin, seamless sleeves at the close dimensional tolerances demanded for proper tracking and interchangeability.

An electrically conductive, flexible, seamless belt for use in an electrostatographic apparatus can be fabricated by an electroforming process wherein a metal from which the belt is to be fabricated is electrodeposited on a cylindrically shaped form or mandrel which is suspended in an electrolytic bath. The materials from which the mandrel and the electroformed band are fabricated are selected to exhibit differing coefficients of thermal expansion for facilitating removal of the band from the mandrel upon cooling of the assembly. In one electroforming arrangement, the mandrel comprises a core cylinder formed of aluminum which is overcoated with a thin layer of chromium and is supported and rotated in a bath of nickel sulfamate. A thin, flexible, seamless band of nickel is electroformed by this arrangement. An electroforming process and apparatus cation Ser. No. 250,894, filed concurrently herewith and entitled Improved Electroforming Process and Apparatus (D/3768), the disclosure of which is incorporated herein. Said copending application is assigned to the Assignee of this invention.

In order for an electroforming operation to be considered economically attractive, it must provide high throughput on a continuous, sustained basis employing a minimum of plant and equipment requirements. 1n addition, acceptable product must be obtained in high yield.

It has been found that the success of a continuous electroforming process is, in large part, dependent upon the ease of parting of the electroformed belt from the mandrel. Thus, it has been found that a diametric parting gap, i.e., the gap formed by the difference between the average inside electroformed belt diameter and the average mandrel diameter at the parting temperature, must be at least about 8 mils and preferably at least about 10 to l 1 mils for reliable and rapid separation of the belt from the mandrel. For example, at a parting gap of about 6 mils, high incidents of both belt and mandrel damage are encountered due to inability to effect separation of the belt from the mandrel.

The parting gap is dependent upon the macrostress in the belt, the difference in linear coefficients of thermal expansion between the electroformed nickel and the mandrel material and the difference between the plating and parting temperatures, in the following manner:

parting gap AT (a a D S.D/E, 0.008 in.

wherein D is the diameter of the mandrel (inches) at plating temperature; S is the internal stress in the belt (psi) and E is Youngs modulus for nickel; AT is the difference between the plating temperature and the parting temperature and a, a, is the difference in linear coefficients of thermal expansion between the mandrel material (M) and the electroformed nickel (Ni).

In an electroforming process of the type described above, the nickel sulfamate electroforming bath is in a dynamic state with many different and sometimes competing reactions occurring. Needless to say, when fabricating belts for highly exacting specifications, it is critical to maintain uniform, steady state conditions within the electroforming bath despite the dynamic nature of the system.

Accordingly, it is an object of this invention to provide a continuous, stable electroforming process.

It is another object of this invention to provide a process for obtaining uniform, steady state conditions in a dynamic electroforming bath under continuous operating conditions.

1t is still another object of this invention to provide endless, seamless, ductile nickel belts exhibiting relatively high tensile strength, and controlled surface roughness at high production rates.

These as well as other objects are accomplished by the present invention which provides a process for maintaining a continous and stable aqueous nickel sulfamate electroforming'solution adapted to form a relatively thin, flexible endless nickel belt by electrolytically depositing nickel from said solution onto a support mandrel and thereafter recovering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprisl istablishing an electroforming zone comprising a nickel anode and a cathode comprising said support mandrel, said anode and cathode being separated by said nickel sulfamate solution maintained at a temperature of from about 140 to 160 F. and having a current density therein ranging from about 200 to 500 amps/ft";

imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution;

maintaining said solution within said zone at a stable equilibrium composition comprising:

Total Nickel 12.0 to 15.0 oz/gal Halide as NiX- .6H O 0.11 to 0.23 moles/gal H 4.5 to 6.0 oz/gal electolytically removing metallic and organic impurities from said solution upon egress thereof from said electroforming zone;

continuously charging to said solution about 1.0 to 2.0 X 10 moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution;

passing said solution through a filtering zone to remove any solid impurities therefrom;

cooling said solution sufficiently to maintain the temperature within the electroforming zone upon recycle thereto at about to 160 F. at the current density in said electroforming zone; and,

recycling said solution to said electroforming zone.

In a preferred embodiment, the aqueous nickel sulfamate solution is maintained at a temperature of from about F. to about F. within the electroforming zone and the current density therein ranges from about 250 to 350 amps/ft and most preferably, the current density is about 300 amps/ft? The aqueous nickel sulfamate solution is preferably maintained at a stable equilibrium composition within the electroforming zone comprising:

Total Nickel 13.5 to 14.0 oz/gtil Chloride as NiCl .6H O 1.6 to 1.7 oz/gal H 30 5.0 to 5 4 oz/gal weight ratio Chloride as NiCli oH- o 012 i 002 Total Nickel pH 3.8 to 4.1 Surface Tension 33 to 37 dynes/cm Additionally, from about 1.3 to 1.6 X 10 moles of stress reducing agent per mole of nickel electrolytically deposited from said solution is continuously charged to said solution. After removal of any impurities from said solution, the solution is cooled to a temperature sufficient to maintain the temperature within the electroforming zone upon recycle of said solution at about 150 to 160 F.

The present invention will become more apparent from the following discussion and drawing which provides a schematic flow diagram illustrating the nickel sulfamate solution treating loop and the nickel belt electroforming loop in accordance with the abovedescribed preferred embodiment.

As shown in the drawing, a relatively thin, ductile, electrically conductive seamless nickel belt is electroformed by preheating an electrically conductive mandrel, such as a mandrel having an aluminum core and a polished defect free chromium coating, at a preheating station 10. Preheating is effected by contacting the mandrel with a nickel sulfamate solution at about 150 F. for a sufficient period of time to bring the mandrel to about 150 F. Preheating in this manner allows the mandrel to expand to the dimensions desired in the electroforming zone 12 and enables the electroforming operation to begin as soon as the mandrel is placed in the electroforming zone 12. Thereafter, the mandrel is transported from preheating station to an electroforming zone 12. The electroforming zone 12 comprises at least one cell containing an upstanding electrically conductive rotatable spindle which is centrally located within the cell and a concentrically located container spaced therefrom which contains donor metallic nickel. The cell is filled with the nickel sulfamate electroforming solution. The mandrel is positioned on the upstanding electrically conductive rotatable spindle and is rotated thereon. A DC potential is applied between the rotating mandrel cathode and the donor metallic nickel anode for a sufficient period of time to effect electrodeposition of nickel on the mandrel to a predetermined thickness. Upon completion of the electroforming process, the mandrel and the nickel belt formed thereon are transferred to a nickel sulfamate solution recovery zone 14. Within this zone, a major portion of the electroforming solution dragged out of the electroforming cell is recovered from the belt and mandrel. Thereafter, the belt-containing mandrel is transferred to a cooling zone 16 containing water maintained at about 60-75 F. or cooler for cooling the mandrel and the electroformed belt, whereby the belt, which exhibits a different coefficient of thermal expansion than the mandrel, can be readily separated from the mandrel. After cooling, the mandrel and belt are passed to a parting and cleaning station 18 at which the belt is removed from the mandrel, sprayed with water and subsequently passed to a drier (not shown). The mandrel is sprayed with water and checked for cleanliness before being recycled to preheat station 10 to commence another electroforming cycle.

The electroforming process described hereinabove is adpated for continuous, high production operation. Key factors in the success of such an operation are the composition of the electroformingsolution, the uniformity of the composition, and the stability thereof during long term continuous operation. The process of the present invention enables a continuous, steady state operation to be maintained with resultant high productivity of nickel belts exhibiting a high degree of uniformity with relatively few rejects.

The relatively thin, ductile electrically conductive seamless nickel belts formed in the present invention must have a relatively high tensile strength of from about 90,000 to about 130,000 psi, yet have a ductility of between about 3 to 12 percent. Moreover, in order for the process to operate on a continuous basis, the electroformed belt must have an internal stress of r .000 055 psi to permit rapid parting of the belt from the mandrel. The belt is very thin, typically exhibiting a thickness of about 0.005 inches. In order to be suitable for use as the substrate for the image retention surface in an electrostatographic apparatus, it is important that the belt exhibit a high degree ofthickness uniformity and a controlled degree of surface roughness. Generally, the surface roughness exhibited by the belt ranges from about 10 to 80 microinches, RMS and preferably, exhibits a surface roughness of from about 30 to.50 microinches, RMS.

In order to maintain a high production rate and consistently obtain useful product meeting such stringent requirements, it has been found necessary in the present invention to employ a nickel sulfamate electroforming solution at very high current densities as compared to current densities normally employed in nickel sulfamate electroforming processes. Generally, the current densities employed in the present invention range from about 200 to 500 amps/ft preferably, the current density is about 250-350 amps/ft and most preferably about 300 amps/ft. In order to minimize plant and equipment cost at optimum throughput, it is considered preferable to operate at both high current densities and high current concentrations. Generally current concentrations for economic operation can range from about 5 to 15 amps/gal.

At the high current density and high current concentration employed in the present invention, a great deal of heat is generated in the electroforming solution within the electroforming cell. This heat must be removed in order to maintain the solution temperature within the cell in the range of about to 160 F., and preferably at about to F. At temperatures below 140 F., there is insufficient temperature differential for the particular materials employed to effect a parting gap between the belt and the mandrel sufficient to allow easy recovery of the belt. To further lower the temperature within the cooling zone 16 to provide an adequate temperature differential would necessitate additional and expensive cooling or refrigeration equipment. At temperatures above about 160 F., hydrolysis of the nickel sulfamate solution occurs underthe acid conditions maintained in the solution resulting in the generation of Nl-l, which is detrimental to the process as it increases tensile stress and reduces ductility in the nickel belt.

Because of the significant effects of both temperature and solution composition on the final product as discussed hereinafter, it is necessary to maintain the electroforming solution in a constant state of agitation thereby substantially precluding localized hot or cold spots, stratification and inhomogeneity in composition. Moreover, constant agitation continuously exposes the mandrel to fresh solution and, in so doing, reduces the thickness of the cathode film thus increasing the rate of diffusion through the film and thus enhancing nickel deposition. Agitation is obtained by continuous rotation of the mandrel and by impingement of the solution upon the mandrel and cell walls as the solution is circulated through the system. Generally, the solution flow rate across the mandrel surface can range from about 4 to 10 linear feet/second. For example, at a current density of about 300 amps/ft with a desired solution temperature range within the cell of about 150 to 160 F., a flow rate of about 15 gal/min of solution has been found sufficient to effect proper temperature control. The combined effect of mandrel rotation and solution impingement assures uniformity of composition and temperature of the electroforming solution within the electroforming cell.

At the high current densities and high production rates employed in the present invention, it has been found that conventional nickel sulfamate bath compositions are unsuitable and do not result in the obtainment of a continuous, stable operation. It has been found, however, that certain aspects of the solution composition and certain relationships between the components thereof are necessary in order to maintain a stable solution and a continuous operation. Specifically, it has been found necessary for continuous, stable operation to maintain the composition of the aqueous nickel sulfamate solution within the electroforming zone as follows:

Total Nickel 12.0 to 15.0 oz/gal Halide as NiX GH O 0.1 l to 0.23 moles/gal H;,BO;, 4.5 to 6.0 oz/gal and to continuously charge to said solution about 1.0 to 2.0 X moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution.

Preferably, for continuous, stable operation with high throughput and high yield of acceptable belts, the nickel sulfamate solution is maintained at an equilibrium composition within the electroforming zone comprising:

Tolal Nickel 13.5 to 14.0 oz/gal Chloride as NiCl .6H O 1.6 to 1.7 oz/gal H BO 5.0 to 5.4 oz/gal weight ratio Chloride as NiCl i6H O 0.12 i 0.02

Total Nickel pH 3.8 to 4.1 Surface Tension 33 to 37 dynes/cm" Additionally, from about 1.3 to 1.6 X 10' moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution is continuously charged to said solution.

it has heretofore been considered preferable to employ little or no metal halides in conventional nickel sulfamate electroforming solutions because they are known to contribute to tensile stress. However, at the very high current densities employed in the present invcntion, the inclusion of a metal halide, generally a nickel halide such as nickel chloride, nickel bromide or nickel fluoride and preferably, nickel chloride, has been found necessary in order to avoid anode polarization. The inclusion of sufficient nickel halide to avoid anode polarization as evidenced by gradually increasing pH was found, however, to impart sufficient tensile stress to the electroformed belts to significantly interfere with the parting of the belt from the mandrel. Although increases in tensile stress can be overcome by inclusion of a stress reducing agent such as sodium sulfobenzimide, the inclusion of a stress reducing agent in excessive amounts was found to result in a high level of surface roughness on the electroformed belts which essentially precludes use of a major amount of such belts as a substrate for image retention surfaces in electrostatographic apparatus.

it has been found, however, that maintaining a high total nickel content of from 12.0 to 15.0 oz/gal and preferably, from 13.5 to 14.0 oz/gal enables a high output of acceptable belts to be obtained on a continuous basis. Surprisingly, it has been found that operation in this range results in a reduction in the reject rate for surface flaws from greater than 50 percent to about 5 percent. Also, it has been found that in the presence of a stress reducing agent, at total nickel concentrations above about 14.0 oz/gal, surface roughness tends to increase; whereas, at total nickel concentrations below about 13.5, an increase in surface flaws is encountered. At the higher nickel concentrations employed in the present invention, relatively high halide concentrations are also required in order to substantially eliminate anode polarization. Since the inclusion of halide is generally undesirable because of accompanying increases in tensile stress, a minimum of halide consistent with reduction of anode polarization is desired. It has been found in a preferred embodiment of the present invention that under steady state conditions, the minimum effective chloride concentration is obtained by maintaining a constant weight ratio of chloride (NiCl .6- H O) to total nickel of 0.12 i 0.02. When this ratio is exceeded, an increase in tensile stress in the belt is obtained. More importantly, however, when this ratio is exceeded, it has been found that at the high voltages necessary for the high current densities employed, corrosion of the cell apparatus is encountered. Operation below this ratio results in unstable operation as marked by poor anode efficiency and dropping pH. Once steady state operation is achieved, it is relatively easy to maintain a constant chloride/total nickel ratio. Over prolonged periods of operation, the chloride concentration tends to decrease slightly because of drag out; whereas, the nickel content tends to increase slightly because of actual differences in electrode efficiencies, although this is offset to an extent by drag out. Accordingly, intermittent addition of chloride (NiCl .6H O) can be made when periodic solution analysis indicates any alteration in the ratio. Since these alterations are slight and occur only after long periods of continued operation, they do not affect steady state operation.

It has also been found that for successful steady state operation and for continuous, high production rates of nickel belts exhibiting essentially constant hardness and tensile strength, the stress reducing agent must be added to the electroforming solution in proportion to the amount of nickel electrolytically deposited from said solution. Advantageously, this can be accomplished by continuously charging to said solution from about 1.3 to 1.6 X 10 moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution. The addition of a stress reducing agent often is associated with the generation of organic degradation products which interfere with the electroforming process and may require periodic shut down for purification purposes. In the present invention, however, the addition of a stress reducing agent is so minute that the equilibrium concentration of organic degradation product is, in essence, inert in the process and does not interfere therewith in any way. In fact, the minute concentration of organic degradation products is a principal feature of the present invention for maintaining a continuous electroforming operation. Suitable stress reductin agents are sodium sulfobenzimide (saccharin), 2-methylbenzenesulfonamide, benzene sulfomate,

naphthalene trisulfomate, and mixtures thereof. Saccharin has long been known as being effective in reducing the stress in electrodeposits (as well as grain refining). in the present invention, it has been found possible to use saccharin effectively at extremely low concentrations. Furthermore, a principal degradation product of saccharin, 2-methy1benzenesulfonamide (2- MBSA), has been found nearly as effective as saccharin itself in controlling stress. Still further, saccharin and 2-MBSA together form a system which tends to mask or minimize the effects of temporary, independent fluctuations in the levels of either component.

The levels of each are best controlled by continuously adding saccharin at the rate per mole of Ni plated set'forth hereinabove and allowing the levels to tend toward their steady state concentrations. The steady state concentrations will be determined by apparent first order reaction rates, as will the rate of approach to steady state.

The steady state concentration of each component is a function of current density, temperature, agitation and current concentration. At steady state, however, the effects of the two components will be independent of concentration and will only be a function of addition rate. Preferably, the weight ratio of Z-MBSA/saccharin at steady state in the electroforming solution ranges from about 2:1 to about 3:1.

The pH of the electroforming solution is also important with respect to both yield and the physical properties of the nickel belt. The pH for successful continuous operation can range from about 3.8 to 4.1. At a pH greater than about 4.1, surface flaws such as gas pitting increase. Also internal stress increases and interferes with parting of the electroformed belt from the mandrel. At a pH less than about 3.9, the metallic surface of the mandrel can become activated, especially when a chromium plated mandrel is employed, thereby causing the metal electroform to adhere to the chromium plating and thus seriously interfere with the operation of the process. Low pH also results in lower tensile strengths ini the final belt which increases the incidence of mechanical damage because of the weaker nature of the belt. The pH level can be maintained by addition of an acid such as sulfamic acid, when necessary.

It has been found that the pH can be essentially maintained within the range set forth above by maintaining a steady state concentration of buffering agent in the solution, generally boric acid (H 80 within the range of 5.0 to 5.4 oz/gal. It has been found that as the boric acid concentration goes below about 5.0, pH control is lost and an increase in surface flaws is observable. lf the boric acid concentration exceeds 5.4, there is sufficient boric acid present to result in precipitation thereof in any localized cold spots, thereby interfering with the process.

It has been further found that control of the surface tension of the nickel sulfamate solution is necessary in order to substantially reduce surface flaws, especially pitting in the nickel belt. The surface tension of the solution can range from about 33 to about 37 dynes/cm in order to assure a high rate of production with minimum rejects because of surface flaws. It has been found that the surface tension of the solution can be maintained within this range by maintaining a steady state concentration of an anionic surfactant such as sodium lauryl sulfate, Duponol 80, a sodium alcohol sulfate, Petrowet R, a sodium hydrocarbon sulfonate (said latter two surfactants being available from E. l. du Pont de Nemours & Co., 1nc.), and the like, ranging from to 0.014 oz/gal within the solution, and preferably, by maintaining a steady state concentration of from 0 to 0.007 oz/gal of surfactant therein. The amount of surfactant required will vary with the quality of water used. It is considered preferably to employ deionized water throughout the process of this invention. The surfactant can be added continuously or periodically to sump 22 via line 34 to maintain the desired surfactant concentration in the electroforming solution.

A typical electrolytic cell which can be employed in the present invention comprises a tank containing a rotary drive means including a mandrel supporting drive hub centrally mounted thereon. In addition, to providing a rotating drive for the hub and a mandrel supported thereon, the drive means provides a low resistance conductive element for conducting as relatively high amperage electrical current between the mandrel and a power supply. The cell, during the electroplating process, is adapted to draw, for example, a peak current of about 3,000 amperes DC at a potential of about 18 volts. In this manner, the mandrel effectively comprises the cathode of the cell. An anode electrode for the electrolytic cell comprises an annular shaped basket containing metallic nickel which replenishes the nickel electrodeposited out of solution. The nickel used for the anode comprises sulfur depolarized nickel. Suitable sulfur depolarized nickel is available under the tradenames SD Electrolytic Nickel and S Nickel Rounds from International Nickel Company. The nickel may be in any suitable form or configuration. Typical forms include buttons, chips, squares and strips. The basket is supported within the cell by an annular shaped basket support member which also supports an electroforming solution distributor manifold or sparger which is adapted to introduce the electroforming solution to the cell and effect agitation thereof. A relatively high amperage current path with the basket is provided through a contact terminal which is attached to a current supply bus bar. I

In order to maintain a continuous steady state operation the nickel sulfamate electroforming solution is continuously circulated through a closed solution treating loop as shown in the drawings. This loop comprises a series of processing stations which maintain a steady state composition of the solution, regulate the temperature of the solution and remove any impurities therefrom, thereby assuring the required conditions within the electroforming cell 12.

The electroforming cell 12 contains one wall thereof which is shorter than the others and acts as a weir over which the electroforming solution continuously overflows into a trough as recirculating solution is continuously pumped into the cell via the solution distributor manifold or sparger along the bottom of the cell. The solution flows from the electroforming cell 12 via a trough to an electropurification zone 20 and a solution sump 22. The solution is then pumped to a filtration zone 24 and to a heat exchange station 26 and is then recycled in purified condition at a desired temperature and composition to the electroplating cell 12 whereupon admixture with the solution contained therein, the steady state conditions set forth above are maintained on a continuous and stable basis.

The electrolytic purification station 20 is provided for removing dissolved metallic impurities from the nickel sulfamate solution prior to filtering. A metal plate of steel or preferably, stainless steel, can be mounted in station 20 to function as the cathode electrode. Anodes can be provided by a pluralityof anode baskets which comprise tubular shaped metallic bodies, preferably titanium, each having a fabric anode bag. A DC potential is applied between the cathodes and the anodes of the purification station from a DC source.

The electropurification station 20 includes a wall thereof which extends coextensively with a wall of the solution sump zone 22 and functions as a weir. The electroforming solution flows from electropurification zone 20 into the solution sump zone 22 via this weir.

The quantity of electroforming solution circulated within the closed loop described herein is maintained relatively constant. Replenishment of solution which is carried away by the mandrel when removed from the electroforming cell and water which is lost through evaporation is provided. The solution can be replenished by the automatic addition of de-ionized water from a source 28 and/or by recycling solution from the nickel rinse zone 14 to sump 22 via line 30. Sensors can be positioned in sump 22 adapted for automatically signaling a low level of solution therein and causing the operation of pumps which pump de-ionized water and- /or rinse solution to sump 22. Additionally, a pH meter can be positioned in sump 22 for sensing the pH of the solution and for effecting the addition of an acid such as sulfamic acid when necessary to maintain essentially constant pH. The continuous addition of stress reducing agents as described hereinabove can be effected at sump 22 via line 32. Also, control of the surface tension of the solution can be maintained by continuous addition of surfactant to the sump via line 34. In this manner, all component additions or make up are made at the sump 22 thereby enabling maintenance of a homogeneous solution at a steady state equilibrium composition within the electroforming cell 12.

The solution which has been electrolytically purified can contain undissolved micron sized solids and sludge from the anodic dissolution of the nickel which must be removed prior to return to the electroforming cell 12. This solution is pumped from the sump tank 22 to a filter station 24 which removes essentially all of the undissolved solids from the solution.

As indicated hereinabove, the temperature of the electroforming solution must be maintained within a desired range in order to provide a desired surface smoothness and uniformity in the electroformed belt. The electroforming solution which flows from the cell is raised in temperature due to the flow of relatively large currents therein and accompanying generation of heat in the electroforming cell. Means are provided at the heat exchanging station 26 for cooling the electroforming solution to a lower temperature. The heat exchanger can be of conventional design and receives a coolant such as chilled water from a cooling or refrigerating system (not shown). The electroplating solution which is cooled in the heat exchanger means can be successively pumped to a second heat exchanger which provides for increasing the temperature of the cooled solution to within relatively close limits of the desired temperature. The second heat exchanger can be steam heated by steam derived from a steam generator (not illustrated). The first cooling heat exchanger can, for example, cool the relatively warm solution from a temperature of 150 F. or above to a temperature of about 140 f. The second warming heat exchanger will heat the solution to a temperature of 140 F. plus or minus 2 F. In addition, the heat exchange station 26 is provided for heating the solution to the operating temperatuers on startup of the system and upon the addition of replenishment solution to the system. The efflux from the heat exchange station 26 is pumped to the electroforming cell 12 where, upon admixture with the solution present within the cell, steady state conditions of both composition and temperature are maintained on a continuous basis.

The following examples further define, describe and compare methods for maintaining a continuous and stable nickel sulfamate electroforming solution in accordance with the present invention. Unless otherwise specified, all percentages and parts are by weight.

EXAMPLES 1-14 Table 1 below summarizes the data obtained in Examples 1-14. The data summarized in Table 1 was collected under continuous and sustained operating conditions in which dynamic equilibrium was established in the manner set forth hereinabove. The data represents average values for runs of 1 to 3 months duration in which the process operated continuously 7 hours per day. Data was not taken until after days of such operation since such period of time is normally required to assure the establishment of dynamic equilibrium as evidenced by uniformity of product and process conditions. In cases wherein the process was unstable, e.g., Examples 1 and 8 or wherein belts could not be parted from the mandrel, e.g., Examples 2 and 7, the runs were no longer than a week in duration.

The process employed both the belt forming loop and solution treating loop as described in connection with the drawing. Except as noted in the Examples, operating, chemical and geometric variables were constant and include the following:

Current Density 4-6 linear ft/sec solution flow over the cathode surface Surface Tension 33-37 dynes/cm" H BO 5.0-5.4 oz/gal Sodium Lauryl Sulfate 0.0007 oz/gal Example 1 2 Mandrel Core Al Al Ni oz/gal 10 10 NiCl '6H O oz/gal 0.75 1.2 Plating Temp. (F) T 150 140 AT (T- -T 75 75 Parting Gap (in.) at 0.009 0.003

T (Parting Temp.) Saccharin Concentration 0 0 Mg/L Wt. Ratio Z-MBSA Saccharin Mole Ratio Saccharin Ni Surface Roughness 10- 15 13 17 so (microinches,RMS)

7t Belts Rejectable for 95 60 Surface Flaws Internal Stress, psi 4000 +6000 Tensile Strength, psi 105,000 100,000 Elongation l 10 ('71 in 2 in.) 5 Coatahility of Se on Good Electroform Process unstable; Belts not partable Remarks Ni depleted; pH

dropping Example 3 4 Mandrcl Core Al Al Ni oz/gal l0 l0 ,5 NiCl '6H:Ooz/ga| 1.2 1.2

Plating Temp. (F) T 150 150 AT (T -T.) 5 Parting Gap (in.) at 0.005 0.0065

T (Parting Temp.)

Saccharin Concentration Mg/L Wt. Ratio Z-MBSA Saccliarm Mole Ratio Saccharin 1 Surface Roughness 13 17 13 l7 (microinclies.RMS) i Belts Rejectahle for (10- 85 0 95 Surface Flaws lnternal Stress. psi +4000 +4200 Tensile Strength. psi 85.000 88.000 Elongation 12 13 (7: in 2 in.) Coatability of Se on Electroform Poor Partahility: Poor Partahility; Remarks Poor eleetroformpoor electroforming ing Yields; poor yields; poor photo- Photoreceptor receptor life due life due to adhesto adhesion ion; highly susceptible to mechanical damage Example 5 6 Mandrel Core Al A] Ni 2 10 10 NiCl,'6H O oz/gal 1.2 Plating Temp. (F1 T 50 150 2' 7s 75 Parting Gap (in.) at 0.010 0.008

T, (Parting Temp.) Saccharin Concentration 7 l2 Mg/L Wt. Ratio Z-MBSA L3 1.3

m Mole Ratio Saccharin 15 X 10 3 X [0- Ni Surface Roughness [8 25 2() 28 (microinches.RMS) '7! Belts Rejecthle for ,0 g5 90 Surface Flaws Internal Stress. psi 2()()() T n ng h. p i 110.000 145.000 Elongation 5 4 ('71 in 2 in.) Coatability of Se on G d G d Electroform RcmilrkS Good partability; Fair partability;

poor electroforming poor clectroforming yield; poor photoyield; poor photoreceptor life due receptor life; to adhesion; good marginal flexing flexing and handling and handling propproperties erties.

Exam le 7 8 Mandi el Core Cu Al Ni oz/gal l0 l5 Ni(l. r6H,O oz/gal 1.2 1.2 Plating Temp. (F) T 150 150 AT (T -T.) 100 Parting Gap (in.) at 0.004

T (Parting Temp.) Saccharin Concentration 7 Mg/L Wt. Ratio Z-MBSA Saccliarm Mole Ratio Saccharin -5 X 1 Surface Roughness 18 25 (microinches.RMS) 7r Belts Rejectable for 65 Surface Flaws Internal Stress. psi 1800 Tensile Strength. psi 105. Elongation ('71 in 2 in.) Coatahility of St: on Go Eleetroform Remarks Belts not partable Process unstable;

Ni depleted. pH

dropping Example Mandrel Core Ni oz/gal NiCl '6H O oz/gal Plating Temp. (F) T AT (T -T Parting Gap (in.) at T (Parting Temp.) Saccharin Concentration Mg/L Wt. Ratio Z-MBSA Saccharin Mole Ratio Saccharin Ni Surface Roughness (micr0inches.RMS) Belts Rejectable for Surface Flaws lnternal Stress. psi Tensile Strength. psi Elongation (71 in 2 in.) Coatability of Se on Electroform Remarks Example Mandrel Core Ni oz/gal NiCl '6H-,O oz/gal Plating Temp. (F) T AT T Parting Gap (in.) at

T (Parting Temp.) Saccharin Concentration Mg/L Wt. Ratio Z-MBSA Saccharin Mole Ratio Example Mandrel Core Ni uz/gal NiClyhH o oz/gal Plating Temp. (F) T 11 AT (T.,-T,) Parting Gap (in.) at

T, (Partin Temp.) Saccharin oncentration t Wt. Ratio Z-MBSA Saccharin Mole Ratio Saleqc harin Surface Roughness (microinches.RMS) Belts Rejectable for Surface Flaws Internal Stress. psi Tensile Strength. psi Elongation in 2 in.) Coatability of Se on Electroform Remarks Poor Partahility Al Al l5 15 1.75 1.75 150 150 75 75 0.006 0.01

Good Poor Se frosts Poor Partahility; Excellent electroforming yield; poor photoreceptor life Excellent partahil ity; low coating yield; excellent adhesion: poor substrate life due to low ductility l l 12 Al Al 13.5 13.5 1.60 1.60 150 150"- 75 75 0.006 0.008

Good Good Fair Partability: good electroforming yield; fair photoreceptor life due to adhesion l3 14 Al al 13.5 1.60 1.60 150 75 75 00 l 0 0.012

Good I Good Good Partability; good electroforming yield; fair photoreceptor life as to adhesion; fair flexing and handling properties Excellent partahility; excellent electroforming yield excellent photoreceptor life; excellent flexing and handling properties It can be seen from the data in Table 1 that only when the equilibrium concentration of the electroforming solution was maintained within the limits of the present invention were electroformed nickel belts obtained on a continuous basis which were suitable for use as con--' 5 ductive substrates for photosensitive image retention surfaces.

Example In Examples 1 14, the chromium surfaces of the 10 ness range of 10-12 microinches, RMS. l5

C. Tank finish chromium with a surface roughness range of 12-20 microinches, RMS.

ln Examples 1 through 7, inclusive, wherein the electroforming solution contained 10 oz/gal total nickel, all

instances of high percentages of rejected belts for sur- 0 face flaws were found to be associated with the use of Class B mandrels; whereas all instances of relatively low reject percentages were associated with the use of; Class A and Class C mandrels. Most likely, this is due to effects caused by the angular surface typical of; ground finished chromium mandrels as-compared to the smoother, mound-like surface characteristic of tank finish chromium mandrels.

Surprisingly, as seen in Examples 8 through 14, inclusive, when the total nickel concentration was increased to 13.5 and 15.0 oz/gal, not only did the percent of re-= jected belts for surface flaws dramatically drop, but also no differences in the reject percentage could be associated with any particular class of mandrels.

This example clearly demonstrates that surface flaws attributable to replication of the mandrel surface are significantly reduced on an overall basis and effectivelyf eliminated with respect to variations between different classes of mandrels by operating in accordance with the' 40 present invention. Moreover, it is significant to note that the surface roughness of the electroformed belts is dependent upon the equilibrium chemistry of the electroforming solution and is entirely independent of the surface roughness of the mandrel surface.

Although specific materials and conditions were set. forth in the above exemplary processes for preparing seamless electroformed nickel belts in accordance with the present invention, these are merely intended as illustrations thereof. Various other additives, concentra covering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprising:

establishing an electroforming zone comprising a sulfur depolarized nickel anode and a cathode comprising said support mandrel, said anode and cathode being separated by said nickel sulfamate solution maintained at a temperature of about to F. and having a current density therein ranging from about 200 to about 400 amps/ft imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution;

maintaining said solution within said zone at a stable equilibrium composition comprising:

Total Nickel l2.0 to l5.0 oz/gal Halide as NiX,'6H O ().l l to 0.23 moles/gal H 30 4.5 to 6.0 oz/gal electrolytically removing metallic and organic impurities from said solution upon egress thereof from said electroforming zone:

continuously charging to said solution about 1.0 to 2.0 X 10 moles of a stress reducing agent per mole of nickel electrolytically deposited from said solution;

passing said solution through a filtering zone to. re-

move any solid impurities therefrom;

cooling said solution sufficiently to maintain the temperature within the electroforming zone upon recycle thereto at about 140 to 160 F. at the current density in said electroforming zone; and

recycling said solution to said electroforming zone.

2. Process as defined in claim 1 wherein the stress reducing agent is sodiumsulfobenzimide.

3. Process as defined in claim 1 wherein the stress reducing agent is a mixture of sodium sulfobenzimide and electroforming solution ranges from about 3.9 to 4.05.

5. Process as defined in claim 1 wherein the surface tension of the nickel sulfamate solution ranges from surfactant to said solution in an amount up to about,

0.014 oz/gal.

7. Process as defined in claim 6 wherein the surfactant is sodium lauryl sulfate.

8. Process as defined in claim 1 wherein the electroforming solution is prepared with de-ionized water.

9. Process as defined in claim 1 wherein the nickel belt obtained exhibits an internal stress of r i p r l a surface roughness of from 10 to 80 microinches, RMS, a tensile strength ranging from about 90,000 to 130,000 psi and a ductility of from about 3 to 12 percent.

10. A process for maintaining a continuous and stable aqueous nickel sulfamate electroforming solution adapted to form a relatively thin, ductile, seamless nickel belt by electrolytically depositing nickel from said solution onto a support mandrel and thereafter recovering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprising:

establishing an electroforming zone comprising a sulfur depolarized nickel anode and a cathode com- 17 prising said support mandrel, said anode and cathode being separate by said nickel sulfamate solution maintained at a temperature of about 150 to 160F. and having a current density therein ranging from about 250 to about 350 amps/ft imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution; maintaining said solution within said zone at a stable equilibrium composition comprising:

Total Nickel 13.5 to 14.0 oz/gal Chloride as NiCl -6H O 1.6 to 1.7 oz/gal a a 5.0 to 5.4 oz/gal Chloride as NlCl- 'oH- o 0.12 i 0.02

Total Nickel pH 3.8 to 4.1 Surface Tension 33 to 37 dynes/cm 18 1i."i 'rbbs5"&fih'ed in claim 10 wherein the cur- L rent density is about 300 amps/ft".

12. Process as defined in claim 10 wherein the current concentration ranges from about 5 to 15 amps/ gal.

13. Process as defined in claim 10 wherein the solution flow rate across the mandrel surface ranges from about 4 to 10 linear feet/second. 7

14. Process as defined in claim 10 wherein the stress reducing agent is sodium sulfobenzimide.

15. Process as defined in claim 10 wherein the stress reducing agent is a mixture of sodium sulfobenzimide and Z-methylbenzenesulfonamide.

16. Process as defined in claim 15 wherein the weight ratio of Z-methylbenzenesulfonamide to sodium sulfobenzimide ranges from about 2:1 to about 2.5: 1.

17. Process as defined in claim 10 wherein the surface tension is maintained within said range by charging a surfactant to said solution in an amount up to about 0.014 oz/gal.

18. Process is defined in claim 17 wherein the surfactant is sodium lauryl sulfate.

19. Process as defined in claim 10 wherein the nickel belt exhibits an internal stress of a surface roughness of from about 30 to 50 microinches, RMS, a tensile strength ranging from about 90,000 to 130,000 psi and a ductility of from about 3 to 12 percent.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,844,906 Dated October 29. 1974 In en 0 Raymond E. Bailev and Douqlas A. Kreckel It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 9, line 32, "ini" should be in-.

Column 9, line 66, "preferably" should be prefe rable.

Column 10, line 10, "as" should be -a.

Column 11, line 61, "f" should be -F-.

Column 14, line 10 "Mole Ratio 1.5 x 10'' I Saccharin Ni should be '-Mole Ratio Saccharin l-.5 x l0 Column 14, line 47, under Example 14, "a1" should be Column 14, line 51, "ll A'I -T should be A'I(T T Signed and sealed this 14th day of January 1975.

(SEAL) Attesta McCOY M. GIBSON 1m. c; MARSHALL DANN Attesting Officer Commissioner of Patents 

1. A PROCESS FOR MAINTAINING A CONTINUOUS AND STABLE AQUEOUS NICKEL SULFAMATE ELECTROFORMING SOLUTION ADAPTED TO FORM A RELATIVELY THIN, DUCTILE, SEAMLESS NICKEL BELT BY ELECTROLYTICALLY DEPOSITING NICKEL FROM SAID SOLUTION ONTO A SUPPORT MANDREL AND THERAFTER RECOVERING SAID NICKEL BELT BY COOLING SAID NICKEL COATED MANDREL EFFECTING A PARTING OF THE NICKEL BELT FROM THE MANDREL DUE TO DIFFERENT RESPECTIVE COEFFICCIENT OF THERMAL EXPANSION COMPRISING: ESTABLISHING AN ELECTROFORMING ZONE COMPRISSING A SULFUR DEPOLARIZED NICKEL ANODE AND A CATHODE COMPRISING SAID SUPPORT MANDREL, SAID ANODE AND CATHODE BEING SEPARATED BY SAID NICKEL SULFAMATE SOLUTION MAINTAINED AT A TEMPERATURE OF ABOUT 140* TO 1600 F. AND HAVING A CURRENT DENSITY THEREIN RANGING FROM ABOUT 200 TO ABOUT 400 AMPS/FT2, IMPARTING SUFFICIENT AGITATION TO SAID SOLUTION TO CONTINUOUSLY EXPOSE SAID CATHODE TO FRESH SOLUTION; MAINTAININGS SAID SOLUTION WITHIN SAID ZONE AT A STABLE EQUILIBRIUM COMPOSITION COMPRISING:
 2. Process as defined in claim 1 wherein the stress reducing agent is sodium sulfobenzimide.
 3. Process as defined in claim 1 wherein the stress reducing agent is a mixture of sodium sulfobenzimide and 2-methylbenzenesulfonamide.
 4. Process as defined in claim 1 wherein the pH of the electroforming solution ranges from about 3.9 to 4.05.
 5. Process as defined in claim 1 wherein the surface tension of the nickel sulfamate solution ranges from about 33 to about 37 dynes/cm2.
 6. Process as defined in claim 5 wherein the surface tension is maintained within said range by charging a surfactant to said solution in an amount up to about 0.014 oz/gal.
 7. Process as defined in claim 6 wherein the surfactant is sodium lauryl sulfate.
 8. Process as defined in claim 1 wherein the electroforming solution is prepared with de-ionized water.
 9. Process as defined in claim 1 wherein the nickel belt obtained exhibits an internal stress of
 10. A process for maintaining a continuous and stable aqueous nickel sulfamate electroforming solution adapted to form a relatively thin, ductile, seamless nickel belt by electrolytically depositing nickel from said solution onto a support mandrel and thereafter recovering said nickel belt by cooling said nickel coated mandrel effecting a parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion comprising: establishing an electroforming zone comprising a sulfur depolarized nickel anode and a cathode comprising said support mandrel, said anode and cathode being separate by said nickel sulfamate solution maintained at a temperature of about 150* to 160*F. and having a current density therein ranging from about 250 to about 350 amps/ft2; imparting sufficient agitation to said solution to continuously expose said cathode to fresh solution; maintaining said solution within said zone at a stable equilibrium composition comprising:
 11. Process as defined in claim 10 wherein the current density is about 300 amps/ft2.
 12. Process as defined in claim 10 wherein the current concentration ranges from about 5 to 15 amps/gal.
 13. Process as defined in claim 10 wherein the solution flow rate across the mandrel surface ranges from about 4 to 10 linear feet/second.
 14. Process as defined in claim 10 wherein the stress reducing agent is sodium sulfobenzimide.
 15. Process as defined in claim 10 wherein the stress reducing agent is a mixture of sodium sulfobenzimide and 2-methylbenzenesulfonamide.
 16. Process as defined in claim 15 wherein the weight ratio of 2-methylbenzenesulfonamide to sodium sulfobenzimide ranges from about 2:1 to about 2.5:1.
 17. Process as defined in claim 10 wherein the surface tension is maintained within said range by charging a surfactant to said solution in an amount up to about 0.014 oz/gal.
 18. Process is defined in claim 17 wherein the surfactant is sodium lauryl sulfate.
 19. Process as defined in claim 10 wherEin the nickel belt exhibits an internal stress of 